Scratch and mar resistant PDLC modulator

ABSTRACT

A PDLC modulator is fabricated using at least one of a selection of specially-formulated UV curable organic hard coatings as a protective layer on the exposed side of polyester (Mylar) film. The hard coatings of various related types show good adhesion on a polyester film substrate, superior hardness and toughness, and have a slippery top surface, which minimizes unnecessary wear. The coating as applied on the modulator surface significantly reduces scratch damage on modulators caused by unexpected particles on the panels under test. In addition, the slip surface will reduce stickiness to particles and therefore also reduce the possibility of panel damage.

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BACKGROUND OF THE INVENTION

This invention relates to liquid crystal modulators and moreparticularly to polymer dispersed liquid crystal (PDLC) modulatorstructures. (PDLC may be a mixture of hydroxyl polyacrylate with theTL-series of liquid crystal, where the hydroxyl group is crosslinked bypolyisocyanate.)

PDLC modulators are used to test thin film transistors in the liquidcrystal display (LCD) industry. Modulator lifetime improvement is one ofthe major objectives in PDLC modulator research and development. Due tothe close proximity of components relative to panels under test, PDLCmodulator structures are subject to damage in normal use, which severelycurtails useful life.

An existing 131 mm square PDLC modulator build on BK-7 optical glass isknown wherein the active surface is protected by a thin film (about 6μm) of polyester (such as Mylar film from Dupont of Wilmington, Del.).Due to the softness of the polyester film, its surface is prone punctureand scratch damage, so that the modulator lifetime has become an issue.The major modulator damage modes include 1) pellicle peeling, 2)particle penetration, and 3) scratching. These damage modes render themodulators useless after relatively short periods of time. As use ofPDLC modulators increases, it has become urgent to develop a more robustmodulator structure.

Prior solutions have been developed which have proved inadequate. Twospecific modes of interest are:

Mechanical Polishing of Glass. This technique involves deposition ofdielectric mirror material onto a glass substrate. The glass was thenlaminated onto NCAP (nematic curvilinear aligned phase) material withthe mirror coating in a sandwich contact with the NCAP material. Theglass is then mechanically polished back, leaving a very thin structure.The resulting thin film shows weak mechanical strength (which is thuseasy to break) when the finished modulator is used in an applicablesystem.

Direct Dielectric Mirror Coating. In this technique, dielectric mirrormaterial is deposited by physical vapor deposition directly onto NCAP orPDLC material. While this approach could potentially provide a hard butthin surface on the top of modulator, no data related to modulatordurability is available. Moreover, attempts at fabricating such astructure have faced technical challenges, including poor yield.

What is needed is a dielectric-based PDLC modulator that is resistant todamage and a method of manufacture.

SUMMARY OF THE INVENTION

According to the invention, a PDLC modulator is fabricated using atleast one of a selection of specially-formulated UV curable organic hardcoatings as a protective layer on the exposed side of polyester (Mylar)film. The hard coatings of various related types show good adhesion on apolyester film substrate, superior hardness and toughness, and have aslippery top surface, which minimizes unnecessary wear. The coating asapplied on the modulator surface significantly reduces scratch damage onmodulators caused by unexpected particles on the panels under test. Inaddition, the slip surface will reduce stickiness to particles andtherefore also reduce the possibility of panel damage.

The invention will be better understood by reference to the followingdetailed description in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B are a side cross-sectional view (not to scale) of aPDLC-based modulator head according to the invention.

FIG. 2 is a flow chart of the fabrication process according to theinvention.

DETAILED DESCRIPTION OF THE INVENTION

In accordance with the invention, UV curable organic hard coatings havebeen formulated and applied as a protective layer on the surface ofelectro-optical modulators. The coatings show good adhesion to plastic(Mylar) substrate, ≧3H hardness, superior toughness and slippery topsurface.

As shown in FIG. 1A and in greater detail in FIG. 1B, an organic hardcoating 1 is placed on an otherwise finished modulator. The organic hardcoating 1 comprises a major hard coating 10, has over it a much thinnerslip agent layer 11. Specifically, the coating 1 is applied to apolyester film (such as Mylar) 2, which in turn covers and protects afragile dielectric mirror 3 that is held in place by a water-borneadhesive layer 4 upon the surface of a PDLC layer 5. An insulative layer6 of silicon dioxide (SiO₂) separates the PDLC layer 5 from atransparent electrode 7 of indium tin oxide (ITO) on a glass substrate8, which is typically BK-7 type optical glass. An antireflective coating9 on the opposing side of the substrate 8 rounds out the structure ofthe modulator head as improved.

Organic Hard Coating Formulation and Process:

A special formulation of the organic hard coating 1 constitutes aspecific embodiment of the invention. The organic hard coatingformulation contains (Trademarks and brand names capitalized):

1) 10%˜50% epoxy acrylate, such as type CN-104 (Sartomer Company, Inc.,Exton, Pa.), CN-120 (Sartomer), CN-124 (Sartomer), Ebecryl-600 (SurfaceSpecialties, formerly of UCB Chemicals of Brussels, Belgium, now ofCytec Industries of West Patterson, N.J.), or like epoxy acrylate;

2) 0%˜30% multifunctional urethane acrylate such as Ebecryl-4827(Surface Specialties), CN975 (Sartomer);

3) 10%˜50% multi-functional monomer, such as SR-399 (Sartomer);

4) 0%˜5% adhesion promoter, such as Ebecryl-168 (Surface Specialties),Ebecryl-170 (Surface Specialties), CD-9052, SR-9009, SR-9011, andSR-9012 (Sartomer);

5) 0.5%˜5% silicone acrylate slip agent, such as Ebecryl-350, andEbecryl-1360 (Surface Specialties);

6) 0%˜15% low viscosity diacrylate monomer, such as SR-238 (Sartomer);

7) 0˜40% low viscosity triacrylate such as OTA-480 (SurfaceSpecialties);

8) 0˜10% crosslinkable toughening agent, such as Ricacryl 3100, Ricacryl3500, Ricacryl 3801 (Ricon Resins, Inc., Grand Junction, Colo.), HycarVTBNX 1300×33, and Hycar VTBNX 1300×43 (BF Goodrich, Akron, Ohio), etc.;

9) 0.05˜5% photo-initiators, such as Irgacure-369 (Ciba SpecialtyChemicals, Tarrytown, N.Y.), Irgacure-500 (Ciba), Irgacure-907 (Ciba),SR-1124 (Sartomer), or their combinations.

The hardness and scratch/mar resistances are further improved bycombining the above organic hard coating with up to 70% (based on totalsolids) inorganic fillers such as MEK-ST and MIBK-ST (Nissan ChemicalIndustries, Tokyo, Japan). 0.5˜5% of silane, such as Silquest A-174,A-1120 (GE Silicones—OSI Specialties, Paris, France) is added to promotethe interfacial adhesion between organic resin and inorganic filler.

A solvent such as methyl isobutyl ketone is added to have solid contentbetween 30˜60%.

The above formulation is then coated on the polyester (such as Mylarfrom Dupont of Wilmington, Del.) surface on top of the modulator byusing a doctor blade, wire wound rod or spinner, or the like. Spincoating is preferred, since it yields a favorable coating thicknessuniformity. The coating thickness is controlled at 3˜15 microns by solidcontent of formulation and/or spinning speed. After complete solventevaporation, the coating is cured under UV irradiation.

Properties of the Organic Hard Coating:

The properties of the organic hard coating according to the inventionare as follows:

Improved Surface Hardness and Mar Resistance. Highly crosslinked organiccoating can easily reach 5 H pencil hardness if it is coated on hardsubstrates such as glass. ≧3 H pencil hardness can be reached on Mylar,which only has ˜HB pencil hardness itself. The coating is softer thanthin film transistor (TFT) panel so that the modulator will not damagethe panel in case the modulator touches the panel.

Improved Scratch Resistance. Modified by a crosslinkable slip agent onthe very top surface, its friction to unexpected particles issignificantly reduced. This will also potentially reduce the possibilityof picking up the particles.

Thin Coating. Thicker coating has better scratch resistance and marresistance. However, the thickness will affect the modulator sensitivityand/or air gap. The current invention preferably uses 1˜20 μm, morepreferably 3˜10μm thick hard coating. The modulator sensitivity or airgap tradeoff is negligible.

Good Adhesion on Mylar. The UV curable coatings in this invention havestrong adhesion on Mylar surface. The adhesion can be further improvedby adding adhesion promoters.

Toughness. By adding crosslinkable toughening additives, the highlycrosslinked organic coating shows superior toughness and flexureresistance.

Low Dielectric Constant. The dielectric constants of the organic andorganic/inorganic hybrid hard coatings are in the range of 2.5˜5, whichhas negligible effect on the spatial resolution.

Solvent Resistance. Since the organic coating is highly crosslinked, itshows superior resistance to common solvents such as acetone,isopropanol, and methyl ethyl ketone, etc. used for modulator cleaning.

1 Coating, “2-Layer” Structure. The formulations contain a slip agent.After coating, the slip agent stays at the top surface due to its lowsurface tension, as shown in FIG. 1.

Fabrication Procedure of PDLC Modulator with Organic Hard Coating:

Referring to FIG. 2, which is a partial process flow chart, thefabrication procedure is as follows:

Step A: PDLC Formulation. 7.54 g of Paraloid AU1033 (Rohm and HaasCoatings of Philadelphia, Pa.), 8.10 g of TL-205 liquid crystal, 5.30 gof methyl isobutyl ketone, and 0.66 g of Desmodur N-75 (Bayer AG,Munich, Germany) are added in a glass vial and stirred overnight. 0.1%of Metacure T-12 catalyst (Air Products and Chemicals, Lehigh Valley,Pa.) (based on total formulation) is then added into the homogeneousmixture and stirred for another 10 minutes.

Step B: PDLC/Adhesive Spin Coating and Pellicle Vacuum Lamination. Themixture prepared in Step 1 is filtered with 1 micronpolytetrafluoroethylene (PTFE) filter and spin coated onto a 135.5×135.5mm square ITO glass substrate. The thickness of PDLC coating layer iscontrolled at ˜17-18 microns by spinning recipe. A thin layer (˜1micron) of Neorez R-967 adhesive (Neoresins, Inc., of Waalwijk, TheNetherlands) is coated on top of the PDLC coating, and the two-layercoating is laminated with the dielectric mirror (on a 6 μm Mylarsupport, “pellicle”).

Step C: Hard Coating Formulation preparation. The components are addedand mixed homogeneously in an amber glass bottle. Formulation Examplesare shown in Table 1. TABLE 1 UV Curable Hard Coating FormulationExamples (values in grams) 1 2 3 4 5 6 CN-104 25 25 25 16.7 16.7 16.7SR-399 30 30 30 20 20 20 OTA-480 40 40 40 26.7 26.7 26.7 Ebecryl-1360 22 2 1 1 1 Methyl Isobutyl 80 80 80 53.3 53.3 53.3 Ketone SR-9012 5 5Ricacryl-3500 10 5 MIBK-ST 60 60 60 Silquest-174 1 1 Irgacure-907 2 2 21 1 1 Irgacure-500 3 3 3 3 3 3

Step D: Organic Hard Coating Process. The formulation from Step 3 isthen filtered by 5 μm filter, spin coated on top of modulator pellicleat 700 RPM for 90 s, and then fully cured by UV irradiation. The dryfilm thickness of hard coating is about 5˜6 μm.

The modulators with organic hard coating showed harder/smooth/slipperysurface, significant improved durability, and negligible defectdetection sensitivity tradeoff.

The invention has been explained with reference to specific embodiments.Other embodiments will be evident to those of ordinary skill in the art.It is therefore not intended that the invention be limited except by theappended claims.

1. A method for fabricating a PDLC-based modulator comprising coatingPDLC material directly on an optical block substrate; coating awater-borne adhesive over the PDLC material; laminating a dielectricmirror/pellicle over the PDLC and adhesive; and coating the polyestersurface with a UV curable organic coating; and UV curing the curableorganic coating to an organic hard coating.
 2. The method according toclaim 1 further including formulating the UV curable organic coating asa mixture of multi-functional acrylates, silicone acrylate as a slipagent, and photo-initiators.
 3. The method according to claim 1 furtherincluding formulating the UV curable organic coating as a mixture ofmulti-functional acrylates, silicone acrylate as a slip agent, acrosslinkable adhesion promoter, and photo-initiators.
 4. The methodaccording to claim 1 further including formulating the UV curableorganic coating as a mixture of multi-functional acrylates, siliconeacrylate as a slip agent, a crosslinkable adhesion promoter, a(meth)acrylate terminated toughener, and photo-initiators.
 5. The methodaccording to claim 1 further including formulating the UV curableorganic coating as a mixture of multi-functional acrylates, siliconeacrylate as a slip agent, a (meth)acrylate terminated toughener, nanosized silica as an inorganic filler, and photo-initiators.
 6. The methodaccording to claim 1 further including formulating the UV curableorganic coating as a mixture of multi-functional acrylates, siliconeacrylate as a slip agent, a (meth)acrylate terminated toughener, nanosized silica as inorganic filler, a silane coupling agent, andphoto-initiators.
 7. The method according to claim 1 further includingformulating the PDLC as a mixture of hydroxyl polyacrylate with aTL-series liquid crystal, where the hydroxyl group is crosslinked bypolyisocyanate.
 8. A PDLC-based modulator with a protective coating,said modulator comprising: a coating PDLC material directly on anoptical block substrate; a water-borne adhesive layer coating the PDLCmaterial; a dielectric mirror/pellicle laminated on top of the adhesivelayer; and a cured UV curable organic hard coating on the pellicle. 9.The modulator according to claim 8 wherein the UV curable organiccoating is a mixture of multi-functional acrylates, silicone acrylate asa slip agent, and photo-initiators.
 10. The modulator according to claim8 wherein the UV curable organic coating is a mixture ofmulti-functional acrylates, silicone acrylate as a slip agent, acrosslinkable adhesion promoter, and photo-initiators.
 11. The modulatoraccording to claim 8 wherein the UV curable organic coating is a mixtureof multi-functional acrylates, silicone acrylate as a slip agent, acrosslinkable adhesion promoter, a (meth)acrylate terminated toughener,and photo-initiators.
 12. The modulator according to claim 8 wherein theUV curable organic coating is a mixture of multi-functional acrylates,silicone acrylate as a slip agent, a (meth)acrylate terminatedtoughener, nano sized silica as an inorganic filler, andphoto-initiators.
 13. The modulator according to claim 8 wherein the UVcurable organic coating is a mixture of multi-functional acrylates,silicone acrylate as a slip agent, a (meth)acrylate terminatedtoughener, nano sized silica as inorganic filler, a silane couplingagent, and photo-initiators.
 14. The modulator according to claim 8wherein the PDLC is a mixture of hydroxyl polyacrylate with a TL-seriesliquid crystal, where the hydroxyl group is crosslinked bypolyisocyanate.